A plasma display panel is a display apparatus which contains a plurality of discharge cells, and is constructed to display an image by applying a voltage across electrodes discharge cells thereby causing the desired discharge cell to emit light. A panel unit, which is the main part of the plasma display panel, is fabricated by bonding two glass base plates together in such a manner as to sandwich a plurality of discharge cells between them.
In a plasma display panel, each of the discharge cells which are caused to emit light for image formation generate heat and each thus constitutes a source of heat, which causes the temperature of the plasma display panel as a whole to rise. The heat generated in the discharge cells is transferred to the glass forming the base plates, but heat conduction in directions parallel to the panel face is difficult because of the poor thermal spreading properties of the glass base plate material.
In addition, the temperature of a discharge cell which has been activated for light emission rises markedly, while the temperature of a discharge cell which has not been activated does not rise as much. Because of this, the panel face temperature of the plasma display panel rises locally in the areas where an image is being generated. Moreover, a discharge cell activated in the white or lighter color spectra generate more heat than those activated in the darker color spectra. Thus, the temperature of the panel face differs locally depending on the colors generated in creating the image. These localized temperature differentials can accelerate thermal deterioration of affected discharge cells, unless measures are taken to ameliorate the differences, and are often referred to as “burn-in” or “image-sticking,” depending upon their level of permanency. Additionally, when the nature of the image on the display changes, the location for localized heat generation changes with the image.
Further, since the temperature difference between activated and nonactivated discharge cells can be high, and the temperature difference between discharge cells generating white light and those generating darker colors can also be high, a stress is applied to the panel unit, causing the conventional plasma display panel to be prone to cracks and breakage.
When the voltage to be applied to the electrodes of discharge cells is increased, the brightness of the discharge cells increases but the amount of heat generation in such cells also increases. Thus, those cells having large voltages for activation become more susceptible to thermal deterioration and tend to exacerbate the breakage problem of the panel unit of the plasma display panel. The backlights for LCD displays, such as LED's (light emitting diodes) and CCFL's (cold cathode fluorescent light or lamp), present similar issues with respect to heat generation as do emissive displays, such as PDP's.
The use of so-called “high orientation graphite film” as thermal interface materials for plasma display panels to fill the space between the back of the panel and a heat sinking unit to even out local temperature differences is suggested by Morita, Ichiyanagi, Ikeda, Nishiki, Inoue, Komyoji and Kawashima in U.S. Pat. No. 5,831,374. However, the disclosure appears to be centered on the use of pyrolytic graphite as the graphitic material and makes no mention of the use or distinct advantages of sheets of compressed particles of exfoliated graphite. In addition, the use of a heavy aluminum heat sinking unit is a critical part of the Morita et al. invention. In addition, U.S. Pat. No. 6,482,520 to Tzeng discloses the use of sheets of compressed particles of exfoliated graphite as heat spreaders (referred to in the patent as thermal interfaces) for a heat source such as an electronic component. Indeed, such materials are commercially available from Advanced Energy Technology Inc. of Lakewood, Ohio as its eGraf® SpreaderShield class of materials. The graphite heat spreaders of Tzeng are positioned between a heat generating electronic component and, advantageously, a heat sink, to increase the effective surface area of the heat generating component; the Tzeng patent does not address the specific thermal issues occasioned by display devices.
Graphites are made up of layer planes of hexagonal arrays or networks of carbon atoms. These layer planes of hexagonally arranged carbon atoms are substantially flat and are oriented or ordered so as to be substantially parallel and equidistant to one another. The substantially flat, parallel equidistant sheets or layers of carbon atoms, usually referred to as graphene layers or basal planes, are linked or bonded together and groups thereof are arranged in crystallites. Highly ordered graphites consist of crystallites of considerable size: the crystallites being highly aligned or oriented with respect to each other and having well ordered carbon layers. In other words, highly ordered graphites have a high degree of preferred crystallite orientation. It should be noted that graphites possess anisotropic structures and thus exhibit or possess many properties that are highly directional e.g. thermal and electrical conductivity and fluid diffusion.
Briefly, graphites may be characterized as laminated structures of carbon, that is, structures consisting of superposed layers or laminae of carbon atoms joined together by weak van der Waals forces. In considering the graphite structure, two axes or directions are usually noted, to wit, the “c” axis or direction and the “a” axes or directions. For simplicity, the “c” axis or direction may be considered as the direction perpendicular to the carbon layers. The “a” axes or directions may be considered as the directions parallel to the carbon layers or the directions perpendicular to the “c” direction. The graphites suitable for manufacturing flexible graphite sheets possess a very high degree of orientation.
As noted above, the bonding forces holding the parallel layers of carbon atoms together are only weak van der Waals forces. Natural graphites can be treated so that the spacing between the superposed carbon layers or laminae can be appreciably opened up so as to provide a marked expansion in the direction perpendicular to the layers, that is, in the “c” direction, and thus form an expanded or intumesced graphite structure in which the laminar character of the carbon layers is substantially retained.
Graphite flake which has been greatly expanded and more particularly expanded so as to have a final thickness or “c” direction dimension which is as much as about 80 or more times the original “c” direction dimension can be formed without the use of a binder into cohesive or integrated sheets of expanded graphite, e.g. webs, papers, strips, tapes, foils, mats or the like (typically referred to as “flexible graphite”). The formation of graphite particles which have been expanded to have a final thickness or “c” dimension which is as much as about 80 times or more the original “c” direction dimension into integrated flexible sheets by compression, without the use of any binding material, is believed to be possible due to the mechanical interlocking, or cohesion, which is achieved between the voluminously expanded graphite particles.
In addition to flexibility, the sheet material, as noted above, has also been found to possess a high degree of anisotropy with respect to thermal and electrical conductivity and fluid diffusion, comparable to the natural graphite starting material due to orientation of the expanded graphite particles and graphite layers substantially parallel to the opposed faces of the sheet resulting from very high compression, e.g. roll pressing. Sheet material thus produced has excellent flexibility, good strength and a very high degree of orientation.
Briefly, the process of producing flexible, binderless anisotropic graphite sheet material, e.g. web, paper, strip, tape, foil, mat, or the like, comprises compressing or compacting under a predetermined load and in the absence of a binder, expanded graphite particles which have a “c” direction dimension which is as much as about 80 or more times that of the original particles so as to form a substantially flat, flexible, integrated graphite sheet. The expanded graphite particles that generally are worm-like or vermiform in appearance, once compressed, will maintain the compression set and alignment with the opposed major surfaces of the sheet. The density and thickness of the sheet material can be varied by controlling the degree of compression. The density of the sheet material can be within the range of from about 0.04 g/cm3 to about 2.0 g/cm3. The flexible graphite sheet material exhibits an appreciable degree of anisotropy due to the alignment of graphite particles parallel to the major opposed, parallel surfaces of the sheet, with the degree of anisotropy increasing upon roll pressing of the sheet material to increase orientation. In roll pressed anisotropic sheet material, the thickness, i.e. the direction perpendicular to the opposed, parallel sheet surfaces comprises the “c” direction and the directions ranging along the length and width, i.e. along or parallel to the opposed, major surfaces comprises the “a” directions and the thermal, electrical and fluid diffusion properties of the sheet are very different, by orders of magnitude, for the “c” and “a” directions.
While the use of sheets of compressed particles of exfoliated graphite (i.e., flexible graphite) has been suggested as thermal spreaders, thermal interfaces and as component parts of heat sinks for dissipating the heat generated by a heat source (see, for instance, U.S. Pat. Nos. 6,245,400; 6,482,520; 6,503,626; and 6,538,892), the use of graphite materials has heretofore been independent, and not viewed as interrelated with other components, such as the frame system of display panels.
Conventional display devices typically utilize a thick, heavy metal support member (often a thick aluminum sheet, or set of multiple sheets) to which is attached both the display panel unit and associated electronic components, such as printed circuit boards. Heat passing from these electronic components contributes to uneven temperature distributions created on the panel unit itself, which adversely affects the image presented on the display panels. Typically, the panel unit is attached to the support member using a two-sided adhesive tape material. Alternatively, the panel unit is sometimes attached directly to the support member using a full sheet of thermally conductive adhesive material, which is commonly a particle-filled acrylic or silicone.
In either case, the conventional support member provides both a mechanical function (i.e., for mounting the panel unit and associated electronics), as well as a thermal function (i.e., to help sink and spread heat generated by the panel unit and/or the associated electronics). Accordingly, the support member is typically fabricated from a solid sheet of aluminum, on the order of about 2.0 mm thick. Expressed another way, the conventional display panel having a support member exhibits a support factor of about 440 mm-W/m° K or higher. The support factor is determined by multiplying the thickness of the support member present in the display panel by its in-plane thermal conductivity (thus, a 2.0 mm sheet of aluminum has a support factor of 440 mm-W/m° K, since the in-plane thermal conductivity of the high thermal conductivity aluminum employed is 220 W/m° K). It will be recognized that, since most metals are relatively thermally isotropic, the in-plane thermal conductivity is not substantially different from the through-plane thermal conductivity of the material.
A support member such as this can weigh about 10 pounds or more, and can be expensive and difficult to construct, due to certain flatness requirements, the need for many threaded mounting features for the electronics, and the high cost of high thermal conductivity aluminum sheet. Additionally, a framework (often made from steel or aluminum) is used to add further mechanical support to the support member, and allow for a robust mounting means for attachment of the display panel to a wall bracket or stand unit. Together, the framework and support member constitute a frame system in the conventional display panel.
Conventional display device manufacturers, especially PDP manufacturers, are under extreme pressure to reduce the cost and weight of their existing display solutions, while there has simultaneously been a desire to increase the brightness and luminous efficiency of the panel units. This can mean more power being sent to the display, which increases the thermal load on the system and requires additional heat dissipation capabilities within the display units. Active cooling solutions, such as fans and/or heat pipes, are undesirable due to unreliability, noise, and the fact that they contribute negatively to the cost and weight of the system. In addition to increasing brightness and luminous efficiency of the displays, display manufacturers are also under increasing pressure to produce larger panel sizes, which tends to increase the weight of the frame system (especially the support member) proportionately.
Thus, what is desired is a light weight and cost effective frame system for display devices, especially one which provides enhanced heat transfer capabilities, yet is structurally sound enough to provide both the attachment for the panel units and associated electronics, as well as the structural integrity for mounting and supporting the display device itself. The desired frame system reduces or eliminates the need for a support member, especially one formed of high conductivity aluminum.